This invention relates to a process of preparing silver-containing catalysts and their carriers for the production of ethylene oxide via ethylene oxidation, and to the use of such catalysts to produce ethylene oxide. Ethylene oxide is produced by ethylene oxidation, and carbon dioxide is also formed in a side reaction. The heat of the side reaction is twenty times that of the main reaction and if not removed in time the ethylene oxide produced in the reaction will be further oxidized to carbon dioxide. In order to increase the activity of the catalysts, the sufficient specific surface area of silver particles must be afforded. Therefore, the catalyst carriers are required to have enough specific surface area. However, oversized specific surface area will make the transfer of the reaction heat difficult, aggravate side reaction and decrease the selectivity of the catalysts. In order to offer the catalysts a high selectivity, an ideal pore structure which matches the surface of the catalyst is required so that suitable conditions for heat and mass transfer can be attained and the unwanted side reaction can be suppressed. Since the reaction takes place under nearly diffusion-controlling conditions, the search for carriers with an optimum matching between pore structure and specific surface area has become an important subject in developing silver catalysts having a high selectivity.
The alumina carriers prepared by the process disclosed in U.S. Pat. No. 4,379,134 have a specific surface area of less than one square meter per gram, generally, from 0.2 to 0.6. The volume fraction of the pores having the diameter of from 20 to 100 micrometer does not exceed 10 percent of the total pore volume. However, the alumina carriers prepared by the process of GB Pat. No. 1,465,523 have a wide range of the specific surface area from 0.1 to 60 square meter per gram, the fraction of the pores having the diameter of larger than 0.1 micrometer is only 40 percent of the total volume. The selectivity of the silver catalysts produced from said carriers is better than that of the silver catalysts produced from oversized the carriers having larger specific surface area, but is still not high.
The purpose of the invention is to avoid the disadvantages arising from oversized or undersized pore structure and specific surface area and inadequate pore-size distribution in the prior art. The present invention relates to alumina carriers having a better match between a specific surface area which compliments the pore structure of the and carrier greatly increases the selectivity of the silver catalysts in commercial applications.